1. Field of the Invention
The present invention relates to an ignition coil for an internal combustion engine and, more particularly, to a stick-type ignition coil to be fitted directly in the plug hole of an internal combustion engine.
2. Description of Related Art
As an ignition coil, a stick-type ignition coil is known. It has a rod-shaped central core disposed in a housing, and a primary coil and a secondary coil wound respectively on a primary spool and a secondary spool made of resin. Resin is filled in the housing of the ignition coil as an electric insulator. The insulator not only provides electric insulation among individual members in the housing but also fills clearances between wires of the coils thereby to restrict movement or breakage of the coils which may arise from engine vibrations. As the insulator, a thermosetting resin such as epoxy is used in consideration of the heat resistance. The ignition coil further has a permanent magnet attached to at least one of the two longitudinal ends of the central core to raise a voltage to be supplied to a spark ignition plug.
In this type of ignition coil, the central core contacts not only the resin insulator but also a case member such as a spool enclosing the outer circumference of the central core. The central core and the resin insulator or the case member, as having different thermal expansion coefficients, may repeatedly expand and contract as the surrounding temperature rises and falls. Then, the resin insulator or the case member, as contacting with the central core, especially the resin insulator or the case member contacting the longitudinal end corners of the central core, may crack, which results in defective electric insulation.
When the resin insulator or the case member around the central core cracks, an electric discharge may occur through the cracks between the secondary coil or a high voltage terminal (high voltage side) and the central core (low voltage side). If the discharge occurs between the high voltage side and the central core, the electric insulation between the high voltage side and the central core is broken to lower the voltage to be generated in the secondary coil, thus disabling a generation of desired high voltage.
If the central core and the resin insulator or the case member repeatedly expand and contract due to changes in the temperature, the central core is caused to receive a load in the radial direction and in the longitudinal direction from the resin insulator and the case member due to the difference in the thermal expansion coefficient. Especially when the central core receives the load in the longitudinal direction, the magnetic permeability of the core may drop causing magneto-striction which disables generation of a required high voltage.
It is desired in a stick-type ignition coil to dispose an outer core around the outer periphery of the primary spool and the secondary spool. Since this outer core contacts directly with the insulator in the housing, the outer core and the insulator having different thermal expansion coefficients, may repeat expansions and contractions as the temperature changes. As a result, the insulator contacting with the outer core may crack causing an electric discharge between the secondary coil or a high voltage terminal the outer core. This discharge lowers the high voltage to be applied to the ignition plug.
In another ignition coil disclosed in Japanese Utility Model Publication No. 59-30501, although not a stick-type, the corners of the core are covered by over-coating the surface of the core with an elastomer. This prevents the corners of the core and the insulator made of epoxy resin from coming into direct contact with each other and suppresses cracks in the epoxy resin in the vicinity of the corners of the core. This over coating is not applicable to the stick-type ignition coil, however, because the stick-type is so regulated in its external diameter as to match the internal diameter of the plug hole.